Which of the following acid-base indicators should be used for the titration of NH3 with HBr? Kb of ammonia is 1.8 ×10−5.
1. Methyl red, color change red/yellow at 4.4 < pH < 6.2 correct
2. Any of these is suitable.
3. None of these is suitable.
4. Phenolphthalein, color change colorless/red-violet 8.0 < pH <10.0
5. Neutral red, color change red/yellow 6.8 < pH < 8.0
Why is answer choice 1 correct?
In the titration of a weak base with a strong base, the equivalence point occurs on the acid side of 7.0. That tells you that 5,4,3, and 2 must be wrong and that the correct answer is #1. However, if you wish to calculate it that can be done, too. The pH at the equivalence point is determined by the salt, in this case NH4Br. The NH4^+ is hydrolyzed to
NH4^+ + H2O ==> H3O^+ + NH3
Ka = (Kw/Kb) = (H3O^+)(NH3)/(NH4^+)
You know Kw = 1E-14.
Kb = 1.8E-5 (for NH3)
(H3O^+) = x = (NH3)
(NH4^+) is about 0.05 if you titrate 0.1M HBr and 0.1M NH3. Work it out. I get (H^+) = 7.45E-6 and the H = about 5.13. That falls just about half-way between the limits of 4.4 and 6.2.
Answer choice 1, "Methyl red, color change red/yellow at 4.4 < pH < 6.2," is the correct choice for several reasons. First of all, during the titration of NH3 with HBr, we expect the pH to change from basic to acidic. Methyl red has a transition range of 4.4 to 6.2, which falls within this expected pH range. So, it will be able to detect the change from the basic, yellow color of NH3 to the acidic, red color of HBr. Secondly, since the Kb of ammonia is given as 1.8 ×10−5, we can expect the equivalence point of the titration to be slightly acidic with a pH less than 7. Methyl red is suitable for detecting this transition as its pH range is within the expected acidic region. Finally, the other indicator options either have transition ranges outside the expected pH range for the titration or have transition ranges that are too broad and may not provide accurate results.
Answer choice 1 (Methyl red, color change red/yellow at 4.4 < pH < 6.2) is the correct acid-base indicator to use for the titration of NH3 with HBr.
In this titration, we are adding a strong acid (HBr) to a weak base (NH3). The reaction between HBr and NH3 produces NH4+ and Br- ions. The pH of the solution will be low due to the presence of the strong acid.
Methyl red is an acid-base indicator that changes color within a specific pH range. In the case of methyl red, it changes color from red to yellow when the pH is between 4.4 and 6.2. Since the expected pH during the titration of NH3 with HBr will be lower than 6.2, the color change of methyl red will be observed.
Therefore, answer choice 1 is correct because it matches the pH range expected during the titration.
Answer choice 1, which states that "Methyl red, color change red/yellow at 4.4 < pH < 6.2" is the correct indicator to use for the titration of NH3 with HBr. To understand why this answer is correct, we need to consider the reaction that occurs during the titration and the pH range at which this reaction takes place.
In the titration of NH3 (ammonia) with HBr (hydrobromic acid), we have a neutralization reaction. Ammonia, being a weak base, reacts with hydrobromic acid, a strong acid, to form ammonium bromide. The reaction equation is:
NH3 + HBr → NH4Br
In this reaction, ammonia accepts a proton (H+) from hydrobromic acid to form the ammonium ion (NH4+). The pH range of this reaction is determined by the dissociation constant of ammonia, known as Kb.
Since ammonia is a weak base, it reacts to a significant extent with water to produce hydroxide ions (OH-) and ammonium ions (NH4+). The equilibrium expression for the reaction of ammonia with water is:
NH3 + H2O ⇌ NH4+ + OH-
The Kb value of ammonia tells us the extent to which this reaction occurs. In this case, Kb = 1.8 × 10^−5.
During the titration, hydrobromic acid is added to ammonia. At the beginning, the solution is basic because ammonia is a weak base. As hydrobromic acid is added, it neutralizes the ammonia, leading to a decrease in pH.
To choose the appropriate acid-base indicator, we need to consider the pH range over which the reaction occurs. The indicator needs to have a color change within this pH range to indicate the completion of the reaction.
Methyl red, the indicator mentioned in answer choice 1, undergoes a color change from red to yellow in the pH range of 4.4 to 6.2. This range corresponds to the pH range where the reaction between ammonia and hydrobromic acid occurs. Therefore, the use of methyl red as an indicator is suitable for this titration.
In contrast, the other indicators mentioned in the answer choices have different pH ranges for their color changes, which do not match the pH range of the reaction. Therefore, they are not suitable for this specific titration.
In summary, methyl red is the correct acid-base indicator for the titration of NH3 with HBr because its color change range (red to yellow) aligns with the pH range (4.4 to 6.2) at which the reaction between ammonia and hydrobromic acid takes place.